Usually with so many replies someone will be well wide of the mark with advice, but all here have offered good direction. There's little question about the grow medium holding too much water, and perlite isn't going to fix it unless there is so much perlite in the mix there is not enough fine material to fill all the spaces between perlite particles. In fact, as you add perlite, o/a aeration decreases until you reach the threshold ratio of coarse material to fine material (this is when the volume of fine soil particles is exactly enough to fill all air spaces between the coarse particles) which is actually the most difficult medium to grow in. Of the images A-G below, D is the most difficult to grow in (just enough fine material to fill all spaces between the large particles, and G is easiest to grow in as long as the particles are not too large. 1/10 to 3/16" would be about perfect. If you can't see the screenshot clearly, Find it Here. Click on screen shot to enlarge. Al

whikxolf, um, my plant is in a house that's 70 degrees F during the day, and 60 over night. i don't think that you need to heat your house up to the conditions of a steamy greenhouse to grow this plant. here's a suggestion... you need to figure out if the plant is drooping because it is wilting, or if the plant is "drooping" because it has decided to place its leaves in that position. plants in the maranta and calathea families are famous for their ability to move and place their leaves. so, if the plant is wilting, the leaves will be soft, abnormally flexible, and you should be able to easily move a leaf around in most any direction. on the other hand, if a leaf feels sturdy and kind of snaps back in place if you try to move it (ie it is turgid), then this is just how the plant wants its leaves. if that's the case, you are simply going to have to get used to the look, or get a different plant. but, more watering will only risk root rot. now, having said that, the "drooping" may be the plant's way to conserve moisture during periods of low humidity. i got my plant months ago, but the heat was already on. mine is "drooping" even a bit more than yours, but it also has three or four new growths... so i know it is happy. it will be interesting to me to see if the leaves have a more upright look once the humidity rises a bit. portland doesn't tend to have high humidity, even during the summer, but it will be higher than during the winter heating season. good luck ps pebble trays and humidifiers seem like something to try as long as you don't overdo it and start to grow mold.

The best soil for Calathea Zebrina is a mixture of one-part moisture-retaining potting soil, two parts perlites for drainage improvement, and two parts of peat to provide organic matter. Zebra plants ideally like to sit in slightly acidic soil. A PH level around 6.5 is considered best for Calathea Zebrina.

it'll spend some time growing roots thru the new soil. just let it be, water carefully, but not too little : water thru the center of the pot, where old roots are so it gets water where it needs it. in general this is high humidity plant. and with such large leaves a lot of water will evaporate from leaves. so try to locate it where you have at least 50% humidity. after you run a shower, put it there overnight - it'll help with keeping the leaves up higher.